Problem Statement

Write a boolean function that tells whether a given integer is prime using Wilson's theorem. By Wilson's theorem, a number p is prime if and only if p divides (p - 1)! + 1. Remember that 1 and all non-positive integers are not prime.

Let's start with the solution:

(defun factorial (n)
  (if (< n 2) 1 (* n (factorial (1- n)))) )


(defun primep (n)
 "Primality test using Wilson's Theorem"
  (unless (zerop n)
    (zerop (mod (1+ (factorial (1- n))) n)) ))